The invention relates to a process for producing polysilicon.
Polycrystalline silicon (polysilicon for short) serves as a starting material in the production of monocrystalline silicon by means of crucible pulling (Czochralski or CZ process) or by means of zone melting (float zone or FZ process). This monocrystalline silicon is divided into wafers and, after a multitude of mechanical, chemical and chemomechanical processing operations, used in the semiconductor industry for manufacture of electronic components (chips).
More particularly, however, polycrystalline silicon is required to an increased degree for production of mono- or polycrystalline silicon by means of pulling or casting processes, and this mono- or polycrystalline silicon serves for manufacture of solar cells for photovoltaics.
The polycrystalline silicon is typically produced by means of the Siemens process. This involves heating thin filament rods (“thin rods”) of silicon by direct passage of current in a bell-shaped reactor (“Siemens reactor”) and introducing a reaction gas containing a silicon-containing component and hydrogen.
The polycrystalline silicon can also be produced in the form of granular silicon with specific fluidized bed reactors (FBR), in which case a reaction gas containing a silicon-containing component is introduced from below through nozzles into a reaction chamber, such that a fluidized bed of granular silicon already present forms and the silicon-containing component reacts at the surface of the granular silicon to give silicon.
The silicon-containing component of the reaction gas is generally monosilane or a halosilane of the general composition SiHnX4−n (n=0, 1, 2, 3; X=Cl, Br, I). Preference is given to chlorosilane or a chlorosilane mixture, particular preference to trichlorosilane.
Predominantly SiH4 or SiHCl3 (trichlorosilane, TCS) is used in a mixture with hydrogen.
Trichlorosilane is preferably obtained via the reaction of metallurgical silicon (MGS—metallurgical grade silicon) with HCl and in subsequent purification in a distillation. Both in Siemens reactors and in FBRs, offgas forms during the deposition, which still contains a considerable amount of silicon-comprising gas. The composition of the gas changes according to the process used. The processing of this offgas is attracting increased industrial attention for reasons of cost.
The prior art discloses methods by which the offgas of the silicon deposition can be processed in principle.
Corresponding cycle processes are described in FIGS. 7 and 8 in O'Mara, B. Herring, L. Hunt, Handbook of Semiconductor Silicon Technology, ISBN 0-8155-1237-6 on page 58.
The offgas from the deposition reactor (Siemens or FBR) is supplied to a twin condensation apparatus, the condensate of which is separated by means of a distillation column into low-boiling and high-boiling components, and the low-boiling components are sent back to the deposition.
The high-boiling components comprise a large portion of silicon tetrachloride (STC), which can be converted to TCS in a conversion apparatus (converter).
The gaseous components of the offgas remaining after the condensation are sent to an adsorption. Here, hydrogen is separated from the other constituents of the gas stream and sent back to the deposition operation. The remaining components are separated in a further condensation into liquid and gaseous components.
The components in liquid form after the condensation are sent to the distillation and used again in the deposition after separation. The gaseous components (labeled “HCl” in FIGS. 7 and 8 in O'Mara) can either be sold as hydrogen chloride (HCl) (cf. FIG. 7 in O'Mara) or reutilized in the initial production (cf. FIG. 8 in O'Mara).
The disadvantage of the processes disclosed in O'Mara is that unwanted substances, for example boron and phosphorus, accumulate in the deposition and thus adversely affect the quality of the silicon deposited.
The “adsorption” already mentioned above is used to purify hydrogen to free it of chlorosilanes still present, and possibly also of HCl.
This involves passing hydrogen contaminated with chlorosilanes and possibly also with hydrogen chloride through a bed of activated carbon at high pressure (pressure between 5 and 20 bar, preferably between 9 and 16 bar) and low temperature (typical order of magnitude T1=20° C.). Instead of activated carbon, it is also possible to use molecular sieves, as described in DE 1 106 298 B. As well as activated carbon, however, it is also possible to use silicon oxide and aluminosilicates as adsorbers; cf. CN 101279178A.
The impurities are physically and/or chemically adsorbed in the activated carbon.
The activated carbon is “unloaded” by means of a regeneration step after the loading by these unwanted substances. This involves lowering the pressure (typical order of magnitude P2=1 bar) and increasing the temperature (typical order of magnitude T2=200° C.).
At the high temperature and the low pressure, the gas constituents which were adsorbed beforehand are desorbed, i.e. released into a purge gas.
Typically, the purge gas (hydrogen) is then cooled together with the impurities in order to separate the components which are liquid at low temperatures from the gaseous components. This is known, for example, from DE 29 18 060 A1.
Thereafter, the adsorber is cooled again and is available again for adsorption.
The gaseous components, which in the prior art comprise predominantly HCl, are sent to initial production (cf. FIGS. 7 and 8 in O'Mara).
The liquid components are sent to a distillation in the prior art, wherein low-boiling and higher-boiling components are separated. The low-boiling components are sent to the deposition, and the higher-boiling components to the conversion.
Typically, several adsorbers each in different phases are used in order to ensure continuous adsorption performance.
The number of adsorbers to be used is determined by the residence time in the corresponding phases of “adsorption”, “heating”, “desorption” and “cooling”.
The longer the “adsorption” phase is compared to the other phases, the fewer adsorbers have to be operated in parallel.
In order to reduce the capital costs for the adsorbers, an attempt is usually made to minimize the times of the “heating”, “desorption” and “cooling” phases.
The effect of the complete recycling of the low-boiling components from the desorption to the deposition operation, which is disclosed in the prior art, is that impurities such as boron and phosphorus in particular (but also Al, As, C) are collected/enriched in the deposition circuit and thus lead to higher concentrations in the silicon produced.
Although all chlorosilanes are obtained for the deposition operation, the quality of the silicon deposited falls.
It was therefore an object of the invention to provide a process in which the concentration of impurities in the deposition circuit is reduced.
It was a further object to be able to optimally utilize all chlorosilanes in the circuit.